The present invention relates in general to the field of data communications and, more particularly, to a method to estimate one or more properties of a transmission line used for digital subscriber line (DSL) technology.
The digital subscriber line (DSL) is a widespread and popular technology that provides high-speed broadband communications to businesses, homes, schools and other consumers over existing telephone lines (conventional twisted-pair wire) without disturbing conventional voice telephony. One type of DSL is asymmetric DSL (ADSL), which normally uses a frequency division multiplexing (FDM) scheme that places the upstream (from customer) and downstream (to customer) communications in separate, non-overlapping, frequency bands. The capacity of a particular ADSL link depends on various transmission channel characteristics or properties of the line (conventional twisted pair wire) between the central office (CO) of the DSL service provider and the customer premises over the used frequency range. These properties are also important for initially setting up DSL service at a customer premises.
These line characteristics or properties can be determined using a training sequence that relies upon bidirectional communications between the DSL transceiver at the CO and the DSL transceiver at the customer premises. Such bidirectional communications are often unavailable because a DSL transceiver is not located at the customer premises prior to installation. As a result, measurement of the transmission characteristics or line properties between a CO and a possible customer premises are performed using a single-ended line test (SELT) by measuring the characteristics from the CO end of the connection.
One SELT method to estimate the length and attenuation of a transmission line involves a technique generally called time domain reflectometry (TDR). With the TDR technique a signal (e.g. a pulse) is transmitted on the line and the received echo signal is recorded. The received signal will contain one or several echoes that could come from: the far-end side of the line, bridged taps, any cable gauge changes etc. A TDR methodology for loop qualification and characterization is described in Galli et al., “Loop Makeup Identification via Single Ended Testing: Beyond Mere Loop Qualification”, IEEE J. Selected Areas in Communications, Vol. 20, No. 5 (June 2002), pp. 923-935. One difficulty with the traditional TDR method is that the reflected pulse can be heavily attenuated and be difficult to detect, as it is hidden by the rather broad outgoing pulse. To avoid this problem, the pulses can be filtered, but the Galli article suggests to instead subtract the outgoing pulse to get a distinct reflected pulse. Another problem with the traditional TDR method is that for short lines, the outgoing and reflected pulses are close to each other and are difficult to separate. For a very long line, on the other hand, the reflected pulse is heavily attenuated and can be hidden in the noise. As a result, only one pulse is observable in some traditional TDR measurements and it is impossible to know if it is a result of the line being very short or very long.
From WO 2006/059175 A1 is a system and method earlier known, which utilizes a transceiver unit comprising a FIR-filter for solving the above stated problems.
Thus, the problem by using echo measurement, such as the described SELT technique, is that it has a number of drawbacks. The distance (time) resolution of the echo measurement is limited both by the hardware transmission bandwidth and regulatory constraints where transmission might be prohibited in certain frequency bands. An example of this is a typical transceiver at the customer premises, ADSL2+ATU-R (ADSL Transceiver Unit Remote Side), which can receive on 2.2 MHz bandwidth but transmit on only 138 or 276 kHz. In many cases, the transmit bandwidth is too small to get good distance resolution in e.g. TDR analysis. Another limiting factor is loop attenuation which may make it impossible to detect the reflected signal, especially for long lines. Yet another limiting factor is noise which can mask the reflected signals. Further, a conventional transceiver unit has to be adapted to perform echo measurement by adding software or circuitry for generation of the test signal, as described in prior art documents above, to be able to perform an echo measurement.